In general, in order to prevent counterfeit, a variety of anti-counterfeit measures are taken for sheets such as bills, coupon tickets, gift tickets. For example, as one of the abovementioned counterfeit measures, micro-printing (of extremely fine characters or patterns) is applied, information of this micro-printing is read, and the read information is compared with valid data, thereby identifying validity thereof (judging authentication). In other words, in the above micro-printing, it is known that specific patterns (moire fringes; moire patterns) are present owing to optical interference because a line width is extremely fine, and further, the moire fringes (moire patterns) are acquired, and the acquired fringes are compared with valid data, thereby identifying validity of sheets.
For example, Japanese Laid-open Patent Application No. 2004-78620 discloses a technique of forming a hidden pattern made up of lines on an information recording object as a sheet, irradiating this hidden pattern with a light source, and sensing reflection light thereof by means of an optical sensor via a check pattern (with a check line pattern formed). In this case, in the optical sensor, lines of the hidden patterns and those of check patterns interfere with one another, thereby making it possible to sense a specific moire pattern, and further, the sensed pattern is compared with a standard moire pattern, thereby judging authentication.
Further, like Japanese Laid-open Patent Application No. 2004-78620 mentioned previously, Japanese Laid-open Patent Application No. 7-306964 discloses a technique of irradiating a sheet having a microprint with light by means of a strobe lighting system, and sensing reflection light thereof by means of an image detector (area sensor) via a moire fringe generator (lattice plate). Specifically, the reflection light from the microprint passes through the lattice plate mentioned above whereby moire fringes may occur. Therefore, after the moire fringes have been sensed by means of the area sensor that is an image detector, if the intensity of a periodic component “fm” thereof exceeds a preset threshold “Th”, it is determined to be affirmative, or alternatively, if the periodic component “fm” fails to exceed the threshold value “Th”, it is determined to be negative.
In the sheet identifying device having an authentication judgment technique mentioned above, a sensor with a resolution higher than that of a conventionally used sensor may be employed in order to enhance precision of judging authentication. In such a case, in the technique disclosed in the publicly known document mentioned above, a filter (lattice plate) having a check pattern is rechecked so that a moire pattern is generated and the filter (lattice plate) according to the recheck needs to be remanufactured, thus making it difficult to restrain higher cost.
Further, in the sheet identifying device for judging authentication of sheets mentioned above, a light emitting element irradiating infrared rays (light emitting element irradiating light with wavelength of infrared-ray bandwidth) is installed in a sheet transfer path, irrespective of a microprint (moire pattern); the sheets to be fed is irradiated with infrared rays; reflection light or transmission light thereof is sensed; and the sensed light is compared with sheet data, thereby occasionally judging authentication. This is a system of judging authentication utilizing wavelength absorption characteristics specific to the print ink applied to sheets.
Incidentally, if bills are exemplified as sheets, under the present circumstances, the bills are prepared with the use of a variety of print inks in countries, thus making it difficult to judge authentication of all of the bills with only one wavelength by means of one identifying device. In other words, a dedicated bill identifying device for each type of bill (for each country's currency) needs to be provided, resulting in higher cost of the bill identifying device. In the future, there may be a case in which a new amount of bill is introduced or a print design is changed, and in the current bill identifying device, there may arise a possibility that precise identification cannot be effected in the future. Thus, a dedicated identifying device needs to be newly manufactured, similarly resulting in higher cost.
The present invention has been made in view of the above-described problem, and aims to provide a sheet identifying device which restrains higher cost and enables judgment of authentication utilizing a microprint formed on a sheet.
Further, the present invention aims to provide a sheet identifying device, which restrains higher cost and enables judgment of authentication, even if a type of sheet to be identified is varied.